There are complex sugars in the wort that can't be utilized by yeast--I
don't know, but suspect, that various yeasts are subtly different about
converting the different sugars. In any event, in practice almost no
beer will ferment out to zero gravity. The ending gravity you get will
depend on the type of malt in the wort, as well as (for those who mash)
how long you mash and at what temperatures.

The temperature correction issue should be approached with extreme
caution. At the suggestion of my friendly dealer-guru, I performed an
experiment where the same wort was checked with the hydrometer fresh out
of the brewpot (~180), and at various temperatures down to the
recommended 60. It turned out that the little correction chart badly
OVERestimated the gravity at the higher temperatures. My advice would be
to check the gravity after the wort has cooled, or, if the purpose of the
check is to see if extracts need to be added to a mash wort, to ladle a
little hot wort into a canning jar, set it in a bowl of cold water or
icewater to cool down, then check the gravity.

I have a question of my own for you folks: are most of you using chlorine
to sanitize your fermenters/carboys/bottles? I have recently had a
couple of batches become infected, even though those same containers are
actually being treated a lot more carefully now than they had been
earlier in my brewing career. I use a hot water rinse, followed by a
dilute bleach-in-water solution. I don't know what I need to correct,
but brewing's too much work to waste a whole batch and all those hours.